I had this question with adsense, and I'm sure that many people who choose very specific niche also think that there might not be very targeted ads for your site.

Well here's a very easy way to find out what ads you will get for your choice of keywords. You don't need approved adsense or even website. Just found this today while reading mails.

You need a Gmail account which is completely free and instant, fast registration. Once your done just log into your account and click "Compose Mail". Now leave everything empty except mail body itself, fill this one with keywords your site (will) have instead. For my example: "BMX, Flatland, Bike, Biking, Freestyle, Extreme, Sport" and click "Save Now". Now open this draft you just created and on the right hand there will be "Sponsored Links" which has the very same ads you would get with adsense while using these keywords.

I'm sure there are a lot more professional ways, but this is free and fast way to do it.

It is very basic and logical way, but I hope someone finds it useful.